1. Field of the Invention
The present invention relates generally to polishing tools, and more particularly to polishing pads for belt-type semiconductor wafer polishing tools.
2. Description of Related Art
Chemical mechanical polishing (CMP) in semiconductor processing removes the highest points from the surface of a wafer to polish the surface. CMP operations are performed on unprocessed and partially processed wafers. A typical unprocessed wafer is crystalline silicon or another semiconductor material that is formed into a nearly circular wafer. When ready for polishing, a typical processed or partially processed wafer has a top layer of a dielectric material such as glass, silicon dioxide, or of a metal, over one or more patterned layers that create local topological features on the order of about 1 .mu.m in height on the wafer's surface. Ideally, polishing smoothes the local features so that the surface of the wafer is flat or planarized over an area the size of a die formed on the wafer. Currently, polishing is sought that locally planarizes the wafer to a tolerance of about 0.3 .mu.m over the area of a die about 10 mm by 10 mm in size.
A conventional belt polisher includes a belt carrying one or more polishing pads, a wafer carrier head which holds a wafer, and a support assembly that supports the portion of the belt under the wafer. For CMP, the polishing pads are sprayed with a slurry, and pulleys drive the belt. The carrier head brings the wafer into contact with the polishing pads so that the polishing pads slide against the surface of the wafer. Chemical action of the slurry and the mechanical action of the polishing pads and abrasives in the slurry against the surface of the wafer remove material from the wafer's surface.
Traditionally, CMP is performed using a planetary CMP apparatus. A planetary polishing apparatus typically includes a rotating polishing table on which polishing pads are mounted. A silicon wafer, held and rotated by a polishing head, is pressed against the surface of the polishing pads in complex motion. Slurry is sprayed or applied onto the surface of the polishing pads by a slurry dispenser. Due to the centrifugal force of the rotating polishing table, the distribution of slurry under the wafer is not entirely uniform, the density of slurry becoming lower toward the periphery of the rotating polishing table.
Recently, linear wafer polishing machines that advantageously address processing requirements of the semiconductor industry have been disclosed. For example, a linear polisher is described in U.S. patent application Ser. No. 08/964,930, entitled "Modular Wafer Polishing Apparatus and Method." In this machine, wafers, held vertically by a wafer carrier head, are pressed against polishing pads attached to a continuous polishing belt. A slurry dispenser is positioned upstream relative to the direction of travel of the polishing belt, as described, for example, in U.S. patent application Ser. No. 08/965,067, entitled "Apparatus for Dispensing Slurry." The slurry needs to be transported on the polishing pads from the point at which it is applied by the dispenser to the point at which the wafer contacts the polishing pads.
Slurry transport is frequently accomplished by providing some non-uniformity in the surface of the polishing pad, such as perforations or grooves in which slurry tends to accumulate. However, any non-uniformity in the polishing pad surface is a potential polishing defect, that is, any location at which the pad does not touch the wafer results in a region of lower removal rate and potential non-uniform polishing. What is a needed is a method of slurry transport in a belt polishing machine that ensures process uniformity.
In addition, in a vertically oriented belt polishing apparatus, such as that described in Ser. No. 08/965,067, wafers may be polished simultaneously on each side of the polishing belt. Thus, on one side of the machine, the polishing belt is moving upward past the wafer and on the other side, the belt is moving downward. It would be desirable to provide a method of slurry transport that overcomes the effect of gravity in a vertically oriented belt polishing machine to ensure an even distribution of slurry on the polishing pads.